1. Technical Field
The present invention relates generally to an alignment table used for automatically locating a work such as a copper-plated laminar plate serving as a base material for a printed wiring board at a predetermined position or for inspecting the work when it is laminated in a laminar structure. More particularly, the present invention relates to an alignment table for an automatic exposing apparatus which assures that figures taken by video cameras can be processed exactly.
2. Statement of the Related Art
When a pattern is formed on a copper-plated laminar plate serving as a base material for a printed wiring plate (hereinafter referred to as a work), the work is coated with solder after completion of a pattern formation or a plurality of works are integrally laminated one above another after completion of the pattern formation, it is required that the work is exactly located at a predetermined position. To meet the requirement, there have been heretofore made many proposals with respect to a method of locating a work in position. One of these proposals is disclosed in Japanese patent application No. 131,954/1988 (corresponding to U.S. Ser. No. 07/355,746 filed on May 23, 1989) which was filed by the common applicant to the present invention. To facilitate understanding of the present invention, the conventional automatic exposing apparatus will briefly be described below.
As shown in FIGS. 5 to 8, the conventional apparatus essentially comprises an aligning section 2 for determining the position of a work 1, an exposing section 3 in which the work 1 is exposed to light beam and a transferring section 4 for transferring the work 1 from the aligning section 2 to the exposing section 3. In detail, the aligning section 2 includes an alignment table designed in a hollow structure to hold the work 1 thereon, support rod 6 disposed below the alignment table 5, a motor drive mechanism 8 for rotating the alignment table 5 about the axis of the support rod 6 and displacing the alignment table 5 above a base 7 in the leftward/rightward direction (in the X-direction) as well as in the forward/rearward direction (Y-direction), a plurality of video cameras 13 for separately photographing via upper and lower transparent plates 9 and 10 attached to the alignment table 5 two or more position determining reference holes 12 (only one of them shown in the drawings) formed on the work 1, a position indexing unit 16 for deriving a central position 14a of each of figures taken by the video cameras 13 (see FIG. 7) by processing the respective figures and moreover calculating a central position 14 of the reference hole 12 by inputting the position of photographing of the video cameras 13 into the position indexing unit 16, a comparative determination unit 18 determining whether or not the work 1 has reached a predetermined position where the work 1 is correctly aligned with a exposing frame 17 when the work 1 is transferred to the exposing section 3 from the central position of the reference hole 12 calculated by the position indexing unit 16 and a drive control unit 19 for controlling the motor driving mechanism 8 so as to derive a satisfactory determination result. As is apparent from FIG. 3, the motor drive mechanism 8 comprises a X-coordinate drive mechanism 20, a Y-coordinate drive mechanism 22 and a THEATA drive mechanism 23.
In FIG. 5, reference numeral 15 designates a position adjusting mechanism for displacing the respective video cameras 13 to a region where photographing can be effected and for generating signals a indicative of positions of the video cameras 13. As shown in detail in FIG. 3, the position adjusting mechanism 15 comprises a X-coordinate drive mechanism 24 for displacing the video cameras 13 in the leftward/rightward direction, a Y-coordinate drive mechanism 25 for displacing the video cameras 13 in the forward/rearward direction and Z-coordinate drive mechanism 26 for displacing the video cameras 13 in the upward/downward direction.
In FIG. 5, reference numeral 28 designates an introduction handler for bringing an unexposed work 1 from an entry stocker 29, reference numeral 30 designates a traverser for transferring the work 1 located in position on the alignment table 5 to the exposing section 3, reference numeral 31 designates a taking-out handler for transferring the exposed work 1 to a taking-out stocker 32, reference numeral 33 designates an exposing lamp, reference numeral 34 designates a mirror and reference numeral 35 designate a suction duct. Further, in FIG. 6, reference numeral 36 designates a suction hole by way of which air is sucked so as to allow the work 1 to be brought in close contact with the upper surface of the alignment table 5.
With the conventional apparatus as constructed in the above described manner, after the work 1 is automatically located in position in the aligning section 2, it is exactly transferred to a predetermined position in the exposing section 3 by the traverser 30. Thus, the conventional apparatus offers advantageous features that it is possible to form a fine pattern and a number of products each having an excellent quality can be produced in a mass production line. However, it has been found from results derived from employment of the aforementioned position determining method that there is a need of improving the following items.
Alignment of a film with the work 2 in the exposing section 3 is carried out before the film is firmly held on the exposing frame 17. As shown in FIG. 6, the upper and lower surfaces of the work 1 including reference holes 12 (only one of them shown in the drawing) placed on the alignment table 5 are fully covered with dry films 38 (mask films), and each of the dry films 38 is curved inwardly in the reference hole 12 in a concave configuration.
Thus, light beam radiated from light sources 39 to reach a concave portion 40 causes a light collecting phenomenon under the effect of a light collecting function of the concave portion 40 (representing the function of a concave lens). This leads to a result that an isolated island-shaped bright spot 43 is generated in a FIG. 42 of the reference hole 12, as shown in FIG. 7. However, this bright spot 43 prevents a figure processing from being smoothly performed, whereby the central position 14 of each of the reference holes 12 is incorrectly detected by the video camera.
The same malfunction as described above takes place also during a solder plating step after completion of the pattern formation. During this step, of course, the lower surface of the work 1 is still not covered with a dry film. Since the upper and lower surfaces of the work 1 are covered with a copper foil having such an excellent surface flatness as given by a polishing operation so as to perform a soldering operation with easiness, reflected light coming from the lower peripheral portion of the reference hole causes a light collecting phenomenon. The collected light generates a plurality of bright spots 44 round the circumferential edge of a FIG. 42 of the reference hole 12. Similarly, the bright spots 44 prevent a figure processing from being performed smoothly.
Additionally, the same malfunction as described above takes place also during a step of placing a plurality of laminar plates each having a predetermined pattern formed thereon, one above another in a laminated structure. During this step, the central position of each of concave or convex recognition marks on the work is recognized but a high intensity of reflected light is generated along a part of the circumferential edge of the recognition mark. This causes bright spots to appear round the circumferential edge of a figure indicative of the recognition mark.